In the SIP-based common messaging system, a session creation method for providing a group messaging service is classified mainly depending on a one-to-one session, an Ad-hoc group session, and a predefined group session. A process of creating the Adhoc group session among these sessions will be described with reference to FIG. 1. Referring to FIG. 1, clients, which are SIP application-based clients that are mounted in user terminals and are responsible for a messaging service, allow users to access the messaging service and serve to create/modify/terminate an SIP session.
To describe a process of establishing a session between multiple clients, it is assumed in FIG. 1 that a client A 100 requests a client B 110 and a client C 115 to establish an Ad-hoc group messaging session. In step 120, the client A 100 generates an INVITE message and sends it to an SIP application server (App Server) 105. The INVITE message may include information about clients to be invited. For example, the client A 100, which applies a conference factory Uniform Resource Identifier (URI) to a Request URI value in an INVITE message, transmits its desired media and session information using the conference factory URI. Therefore, if the Request URI in the INVITE message corresponds to a conference factory URI, the server 105 performs a conference function and a participating function. Accordingly, the server 105 establishes and manages an Ad-hoc group messaging session based on the INVITE message sent by the client A 100.
In steps 125 and 130, the server 105 sends INVITE messages including the media and session information desired by the client A 100 to the client B 110 and the client C 115. After receiving the INVITE messages, the client B 110 and the client C 115 can respond depending on the users' decisions. When accepting a call with the client A 100, the client B 110 and the client C 115 send 200 OK messages with their desired media response information to the server 105 in step 135 or 145. In response, the server 105 sends a 200 OK message to the client A 100 in step 140.
A one-to-one session is established by requesting one member to participate in the session in an Ad-hoc group session establishment method and procedure. Therefore, a one-to-one session establishment method and procedure is identical to the Ad-hoc group session establishment method and procedure.
For establishment of a predefined group session, information about a predefined group is used. Accordingly, upon receiving a session establish request message for a predefined group from a client, the server 105 sends a session establish request message to each member in a group member list acquired by performing a process of acquiring the group member list.